In rising stem or dual action ball valves such as those described in U.S. Pat. No. 4,917,354 a ball formed with an opening extending through it is rotated 90.degree. between closed position wherein it closes the valve and open position wherein it opens the valve. The valve includes a valve stem which is caused to move up and down by a handwheel or other actuator and during such vertical motion, it is rotated. Cooperating cam surfaces on the lower end of the stem and on the upper portion of the ball cause the ball to move into and away from the valve seat during closing and opening of the valve. This action facilitates opening and closing the valve.
When the ball is turned to closed position it seats against a seat ring which is fitted into the valve body surrounding the ball. For secure sealing of the valve when in closed position, an organic substance such as Teflon or Nylon may be used as a component of the seat ring and which yields under pressure sufficiently to conform to the mating surface of the ball, thereby creating an effective seal. However, such organic materials deteriorate when subjected to high temperatures.
Expedients that are presently in use are various forms of metallic seat which require very accurate machining and finishing processes. Because of the rigidity of such materials, the machining and finishing must be done very accurately, and even so wear during use and the effect of high temperature will cause defects in the mating surfaces of the ball (or other closure) and the seat.